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VLT ELECTRONIC CABINET
COOLING SYSTEM
USER MANUAL
VLT-MAN-ESO-17130-1603
Issue: 1.0
Date: 27 Apr. 98
Page: 26of 57
The host acts as the master of the point to point communication, while the controller acts as the slave.
Every time the controller receives a message from the host, it sends back an acknowledge message
which contains the information required by the host, if any.
The controller never takes the initiative of sending a message to the host.
All the messages sent by the controller are acknowledge messages.
Only one message (command ‘C’) causes two acknowledge messages to be sent back. This was
implemented for synchronisation purposes, the second message being synchronous with the sampling
of the temperatures.
A new message to the controller cannot be sent by the host before the acknowledge (or the
acknowledges for the ‘C’ command) to the previous one has been received.
The delay between the reception of a command and the transmission of the acknowledge depends on
the specific command but is normally in the order of a few milliseconds.
Some messages contain numeric values. These values are represented as the corresponding
hexadecimal strings.
The temperatures are represented internally by a 15 bit positive number (normalised format).
The same format is used to represent the temperatures in the serial messages.
To convert a temperature tN from the normalised format to the corresponding value expressed in
Celsius degrees, the following relationship has to be applied:
t[°C] = (tN-5750)/375
The normalised format can represent temperatures in the range -15.+72 °C with a resolution better than
0.003 °C.
For the hex representation the protocol can accept both upper and lower case alphabetic characters,
although the messages sent back by the controller contain always upper case characters.
The character representing the most significant nibble of each byte is transmitted first.
Numeric values (16 and 32-bit long) are transferred with the most significant byte first.
This rule does not apply to the transfer of parameters blocks.
The blocks are organised in 16-bit words and the words are transferred with the least significant byte
first
In addition, blocks having an odd number of bytes (blocks 0 and 2) are expanded with an additional
dummy byte, which is inserted before the last byte.
9.3.1 MESSAGE STRUCTURE
The messages from the host to the controller are structured in fields as follows:
<initiator character><serial address><command><parameters><checksum><terminator character>
where:
the field <initiator> is composed by the ">" character ASCII code: 62
the field <serial address> is composed by two characters specifying the address in HEX format.
This field has been introduced to support the multidrop communication with the RS485 hardware
protocol.
For RS232 interface the field must be always "00".